Transpiration and the Resistance of Stems 347 



Another plant transpired 4.73 grm.; cut, 3.9. Percentage of 

 cavity, 5 5.6. Area of wood, 47. 56. Area of cavity, 26.45. This 

 would necessitate a rise of only 17.5 cm. while the eosin solution 

 reached a height of 43.4 cm., requiring an area of 10. 1 sq. mm. 



A branch of Taxns was tested in the same way. Measure- 

 ments of different sections from the cross section of the stem gave 

 exceptionally uniform results. The area of the wood, 29.3 sq. 

 mm. Percentage of cavity 35.5. Area of cavity, 10.40 sq. 

 mm. Absorption by stem 2.19 grm. The rise therefore would 

 be 20.2 cm. Recorded rise of eosin 31 cm.; necessitating an area 



of 6.77 sq. mm. It should be stated that the above-mentioned 

 tests were made on plants with small transpiring leaf areas, as the 

 amount of absorption indicates. 



It will be seen that the percentages secured in these experi- 

 ments correspond in the main with results obtained by Rb. Har- 

 tig and Sachs and it would appear that there is less opportunity 

 for the introduction of error in this method while in addition the 

 directness of obtaining the results renders the process more avail- 

 able than the specific gravity method. 



It is very evident that the carrying capacity of the water con- 

 duits is greatly in excess of any demands made upon it by the 

 plant and it is reasonable to suppose that the proportions expressed 

 above between the volume of water and the area of cavity of the 

 stems fairly represent the conditions that obtain in these plants. It 

 will be noted that the percentage of air space is not necessarily 

 larger in the more actively transpiring plants. These relations 

 render quite unnecessary the supposition of Vines that the walls 

 under certain conditions may serve as channels in the transmission 



of water. 



Summary 



By means of pressure tests a definite measure of the resistance 

 of a given length of stem to the transpiration current may be 

 obtained and the work actually performed by the plant can be 

 computed. More frequently in the case of small plants by the 

 lifting method of measuring this force a partial estimate of what 

 the plant can do rather than what it is doing is obtained. 



The resistance overcome by the transpiration current is often 

 much higher than can be measured by the suction of the transpir- 

 ing shoot and is subject to extensive variations in different species 



